Probes, specific for defined chromosomal components, facilitate the study of the structure of various functional states of chromatin and chromosomes. Antibodies specific for individual histone fractions are used to study the organization of these proteins, in metaphase and polytene chromosomes, as well as in the transcribed and nontranscribed regions of the genome. Immunochemical techniques are used to study the composition and heterogeneity of purified nucleosomes. Immunofluorescence and immunoelectron microscopy allow visualization of the location of particular histones in chromosomes, in metabolically silent chromatin, and in chromatin actively undergoing DNA replication and RNA transcription. Antibodies against purified non-histone proteins (HMG) have been elicited. These antibodies bind to chromatin and can be used to study the structure-function relationship of these proteins.